Vacuum tube comparator



March 25, l952 w. P. MOLAND ET AL 2,590,116

VACUUM TUBE UOMPARATUR Filed April 1o, 1951 s sheets-sheet 1 I N V ENTCR3 HOW/919D H FEEE/ B :YA/Hmm P. Mo; /7/1/0 {Sheets-Sheet 2 Mardi 25,1952 w. P. MOLAND ET AL VACUUM TUBE coMPARAToR Filed April y10, 1951March Az5, 1952 w. PQMOLAQD Em 2,590,116

VACUUM TUBE COMPARATOR Filed April 10, 1951/ .3 Sheets-Sheet 5 IN VENTORS.

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OTTOE/VV Patented Mar. l225,

voUUMTUBE COMPARATOR Warren iP. Malena, Dorchester, anu 4Hmva'rtl H.`Ferris, Milton, Mass.

Application April 1o, 1951, serial No. 220,324 '5 4'claim-s. (o1.cit-370i (Grantee under) the act ,affilier-geil s, i883, as

amended Apri-1 so, 192s; 37o o. G. 757) The invention describedherein/may be manufactured and used by or for the Government pforgovernmental purposes without payment to us of any royalty thereon. l

This invention concerns the testing of vacuum tubes and more specicallyto a tube voltage and current comparator. y u

In the past, balancing variablevresistor circuits have been used indetermining the performance of vacuum tubes in terms of thetransconductance between pairs of elements therein.

The present invention comprises a saw tooth voltage generator supplyinga succession of plate potentials as a saw tooth voltageor wave to areference tube and to a tube under test, to the control grids of whichthe outputs from three thyratrons that lire in succession during eachsaw tooth wave are applied and means for causing the tube outputs toappear simultaneously on the screen ci a cathode ray tube.

The objects of the present invention comprise the provision of a new andimproved comparator for displaying the characteristics of a pair ofvacuum tubes to the electrodes of which desired potentials are app-lied;to a comparator making its presentation within 'the heating time of thetubes in its circuits; and to an improved .cornparator to be used intube design and test.

An embodiment of the present invention is shown in the accompanyingdrawings wherein:

Fig. l is a block circuit diagram of a vacuum tube comparator thatembodies the present invention;

Fig. 2 is aschematic circuit diagram. of the comparator shown in Fig. l;and

Figs. 3' and l are graphs of illustrative tube performance using thecircuit shown in Fig. 2.

In the circuit shown in Fig. l a saw tooth Voltage generator lilsupplies a saw tooth voltage to the plates of both a standard tube IIand a tube under test I2 and across the horizontal deflection plates I3and t4 of a cathode ray tube I5. A linear step voltage generator I6passes its output to the control grids of both vacuum vtubes II and I2.Output from the tubes II` and I2 is `applied through an electronicswitch I'I to across the vertical deflection plates I8 and I9 of thecathode ray tube I5.

Operatively the generator I supplies voltage to the plates of the vacuumtubes II and I2 and actuates the horizontal sweep of the cathode raytube I5. The vacuum. tubes II and I2 receive upon their control gridsthe output from the linear step voltage generator II. The tubes II andI2 periodically conduct under the iniiuence of the output from thegenerator I6. The outputs from the tubes I l and I2 are applied acrossthe vertical deection plates of the cathode ray tube I5. Comparableconduction patterns for the standard tube II and for the tube I2 undertest then appear for comparison beside each .-'2 other on the screen ofthe cathode ray tube I5, as represem-,te,din'Figs.,-5 and 6 of thedrawings.

`.A.schematic circuit diagram of a vacuum tube `comparator that embodiesthe `present invention is ShQWn in Fig.- -2 of the drawings. In theschematic circuit a saw V tooth of high voltage is generated in `a highvoltage saw tooth voltage generator vZ-Fi and a saw toothr of lowvoltage is gernerated Vin a low voltage saw tooth voltage generator 26.One ofV these voltages at a time is appliedv by a plate voltageyselector switch'l'l through a suitable resistor in an VampliiierselectorZM to an amplifier 39 in an electronic switchil that alsocontains a multivibrator 2,0 of the flip-flop or Eccles-Jordan type. Anabove ground power supply 24 supplies `+300 volts to the movable taps ofthe amplifier gain balance variableresistors 12 and 'I3 in theelectronic switch I'I. y l

yOutpaitirom the linear step voltage generator I6 is passed through a`step voltage selector, 3I to the control grids of the tubes II and I2in a tubeadapt'er 29. |lhe tubes II and I2 have an adjustable screenvoltage applied by switch 32 to their screen grids. The screen voltageswitch is closed on the closing of the tube switches V33 with which itis ganged. Upon the closing of the pairv of switches 33, the comparedtube outputs are passedI to the control grids of amplier tubes 34 and '35 in the amplifier 3D of the electronic switch I1. l One of thev sawtooth voltage generatorsZS illustratively may be designed to generate amax -f imum Voltage'of 300 volts across a. load drawing a peak currentof 60 milliamperes. The other y saw tooth voltage Agenerator 25illustratively may be designed to generate a maximum of 700 volts acrossa load'drawing 5,00 milliainperes peak currentV and hence produces ahigher output voltage than the generator 26. The lower voltagegeneratif2li isV .Supplied withV `-l-500 volts potential ad. the higher voltagegenerator 25 is Supplied With -I-'l'OOVO'lts" potential'.

The low voltage saw tooth generator 26 comprises a relaxation oscillator2| containing the tubes 40, 4Iand 431 The positive potential slipp'l'y O-l`-50,0"V`` `1ts`"is applied to the plate of thyratro'n'tube' throughseri'es'cornected resistors 22 and 23 and directly" tothepflate of thevacuum tube 'I'I.l negative potei'fitialv of -l'05 volts issoppiiedto'tn grid or the thyijatrntu through the adjustable pistevoltage vcenti-oi potentiometer 44' arid" through the resistor of thepotentio'iiieter 44" tothe cs'itliode of the thyratron il() and to theplate of the diode 43. The cathode of the vacuum Atube 4t is"` connectedthrough a resistor at;witnltnecathcdejof the, time fin' arid" with thepiatenr the' diode 43': Tne'cutputfrmte cathode follower tubeklII is`coupled with the amplier input selector 28' through the' capacitor 42`and' is held at xed potential by the action of the diode 3 tube 43. Theamplitude of the saw tooth wave output of the low saw tooth generator 6is controlled by the bias level of the grid of the tube 4D establishedby the adjustment of the movable tap on the adjustable potentiometer 44in the grid-cathode circuit of the thyratron 40. The output from the lowvoltage generator 25 is supplied to the selector 28 and moreparticularly to one terminal of the plate voltage selector switch 21therein.

The high voltage saw tooth generator 25 comprises an oscillator 49 and aconstant voltage cathode follower device of Very low impedancecomprising the tubes 60 and 6I and the transformer 53. The oscillator 49comprises a pair of series connected thyratrons 50 and l53 connected inparallel with a capacitor 59 to ground. A +1,00() voltage potential isapplied in the oscillator 49 through a resistor 58 to the plate of onethyratron tube 50 to connected plates of capacitors 55 and 59 and to thecathode of the diode 56. The +1,000 volt potential also is applieddirectly and in parallel to the plates of the vacuum tubes 60 and 6I inthe constant voltage device. A direct current potential from a battery Iof illustratively 67% voltage is applied across the resistor of avariable potentiometer 52 having a variable tap from which all or partof this voltage is applied as grid bias to the grid of a secondthyratron tube 53. When the lower tube 53 is caused to be fired from thecharge on the capacitor 59 and the setting of the tap on thepotentiometer 52, the potential of the grid of the upper tube 50 tendsto stay xed applying a positive voltage on the grid of the upper tubewhich triggers it. The plate of the tube 53 is connected to the grid ofthe thyratron tube 50 through a resistor 54 and is connected directly tothe cathode of the tube 53. The plate of the thyratron tube 52 iscoupled by capacitor 55 with the cathode of the diode tube 55. The diodetube 56 has potential from a voltage source 51 supplied to its plate andis shunted by a resistor 48 to provide a clamping circuit that maintainsthe voltage supplied by the oscillator 49 above the base line. A chargeimpressed upon the capacitor 59 in the plate circuit of the thyratroncauses both of the series connected thyratrons 50 and 53 to conduct. Thepotentiometer 52 provides a control for the oscillator 49. The battery51 illustratively also may be 671/2 Volts. 56 is coupled in parallelthrough resistors 61 and 68 with the grids of the tubes 60 and 6I.

In the constant voltage device in the voltage generator 25, analternating current source 62 is applied through the transformer 63 tothe cathodes of the tubes and 6I. A center tap on the secondary windingof the transformer 63 is: applied to ground through a resistor 64.Output from the high saw tooth voltage generator 25 is supplied from thecenter tap on the secondary winding of the transformer 63 that isconnected to one terminal of the plate voltage selector switch 21 in theamplifier selector 28.

Within the amplifier input selector 28 the switch arm of the platevoltage selector switch 21 is resistively connected with a desired plu-Irality of resistors of preferred magnitudes arranged in parallel ingroups to be swept by the ganged switch arms of the switches 45 and 4S..The switch arms of the switches 45 and 46 are connected directly andrespectively to the control. grids of amplifier tubes 34 and 35 in theamplifier 304 in the electronic switch I1.

The output from either the high or the low saw The cathode of the tube 4tooth voltage generator 25 or 2B may be selected by the plate voltageselector switch 21 in the amplier input selector 28. The saw toothvoltage so selected is applied as a sweep voltage directly and inparallel to the ungrounded horizontal deection plate I3 and to onevertical deflection plate I9 of the cathode ray tube l5. The saw toothvoltage so selected also is applied through a pair of selectedresistors, such as the resistors 65 and 65 as examples, in the gangedswitches 45 and 46 and through the ganged tube switches 33 in the tubeadaptor 29, together with the outputs from the plates of the vacuumtubes I2 and I I that are being compared, to the conf trol grids of theamplifier tubes 34 and 35 in the amplifier 30 of the electronic switchI1, respectively, throughout. The voltages developed across theresistors 65 and 65 are proportional to the plate currents of the tubesI2 and I I, respectively, being compared.

In the multivibrator 20 of the electronic switch I1 multivibrator tubes19 and 1I are connected in push-pull with resistors in series betweentheir cathodes. The tubes 10 and 1I function as a multivibrator in thatthey alternately cut off the amplier tubes 34 and 35 in the amplifierSil. A variable resistor 69 between the cathodes of the tubes 10 and 1Iserves to adjust the cathode bias on the tube 1I and thereby assists inbalancing the multivibrator 20. The plates of the electronic switchamplifier tubes 34 and 35 receive a potential of +300 volts throughadjustable amplifier gain balancing potentiometer 13 from the powersupply 24. The plates of the tubes 34 and 35 are connected in paralleldirectly with and alternately pass their outputs to the upper verticaldeection plate I8 of the cathode ray tube I5.

The cathode ray tube plates I3 and I9, in addition to being connecteddirectly to the sweep arm of the switch 21, are connected in parallelwith the negative terminal of the power supply 24 and, through ampliergain balancing resistors 12 and 13, to the plates of the amplifier tubes34 and 35.

The plate current variation applied to the vertical deflection plates I8and I9 of the cathode ray tube I5 is that of first one tube and then theother of the tubes II and I2. As a consequence it is possible to comparesimultaneously the merits and the characteristics of the two vacuumtubes from the presentation on the screen of the cathode ray tube I5.

To compare the characteristics of the two tubes II and I2 with variouslevels of grid voltage, plotted as indicated against time in Fig. 3 ofthe drawings, a linear step voltage is generated and is applied to thecontrol grids of the tubes II and I2. In the linear step voltagegenerator l5 a +300 Voltage plate potential is supplied to amultivibrator 15', to a pair of limiters 11 and 18', to a gate circuit19 and to a cathode follower 33'. Output from the multivibrator 15',that may be a common form of free running multivibrator comprising thetubes 15 and 16, is coupled to the two limiter tubes 11 and 1S thatminimizes Variations in the amplitude of the output from themultivibrator 15 to the gate 19 and to a duo-diode 93. The output fromthe limiter tube 10 is passed through a capacitor 14 to the grid of thegating tube 19 in the gating circuit 19. The gating tube 19 conductseach time its bias is overcome by the charge on the capacitor from the+300 voltage passing through the series connected plate resistors BI and82. Each time the capacitor 80 discharges Vplotted along the abscissa. v

of the tubes il and l2 have an adjustable bias and 12.

alsaw tooth voltage is generated. The saw tooth 'voltage output fromkthe gatingtube 19 is passed to the grid of the cathode -follower tube18,3.

A Vcathode follower tube `8S is connected in shunted by diodes 88, 9S`and S2, respectively throughout, with a desired plurality of adjustableynegative voltages applied as `gridbiases to ltheggrids Vcfacorresponding plurality of thyratrons 85, 86 and A plate voltage of+1500 volts is applied in parallel through resistors M5, H6 :and lll' tothe plates of the thyratrons 85, The resistors H5, lili and i-lll areshunted respectively with one-half Wavefrectiiier twin diode tubes es,di and 93 capacitively coupled respectively by capacitors i3d, i3! and132 with the voltage input into the plate of the limiter tube 'il'. rlheresistors lit land l Il preferably are variable resistors, as shown. Thenumber of thyratrons d, 86 and sl with their associated circuits thatare used depends on the number of grid voltage steps desired to beapplied to the control grids of the Jtubes il and l2 `during theapplication of each saw-tooth to the plates thereof;

v'Ifhe bias levels oi the thyratron tubes B5, -iiii and 8'! are soestablished that these tubes are nonconducting in the absence of thesawtooth voltage output from the cathode of the tube 33. With the thyratrontubes '35, 85 8l' nonconducting the voltage drop in the step voltageselector 3l across the series resistors 95 through 99, inclusive, iszero. With the thyratron tubes 85, 86 and 8l conducting, as the positivegoing saw-tooth voltage rises in its application to the grids of thethyratron 'tubes 85, 85 and 81, the firing potential of the ltube S5will be reached rst. The ring of the thyratron 85 causes it to conductthrough a selected part or all of the series connected common cathoderesistors e5 to 99, inclusive, and causing the conducted potential to beapplied through the switch arm and the capacitor lill in parallel tothecontrol grids of both of the tubes Il and l2. This develops the iirststep voltages lill and HU in Fig. 3 of the tubes Il and i2,respectively, as static characteristic curves oi the tubes, with gridvoltages plottedalong the ordinate and time The control grids of fromzero to -30 volts applied thereto through a-resistor H12 shunted by adiode tube 103.

As the saw-tooth voltage continues to rise, the ring potential of thetube 85 will ,be `reached and as its plate current flows through part orall of the common cathode resistors v.S to .99 inclusive, the secondstep voltage lll and Al H in Fig. 3 of the tubes l! and I2 respectively,is developed. In a similar manner, as the saw tooth voltage continues torise, the iiringpotential of tube 31 will be reached and the third stepvoltage `H2 and H2 of Fig. 5 of the tubes Il and I2 respectively, isdeveloped. Fig. 3 represents the graphical presentation of a family-ofstatic characteristic curves for each of the tubes l i As previouslystated, the number of steps may be changed by changes inthe number ofthyratrons 85, 86 and 8l in the linear step voltage generator l5, aspreferred.

In 'Fig. 4 the pairof curves 24 illustratively are at zero volts biasstep; the pair of curves |25 are at -3 volts bias step; and the pair ofcurves |26 are at --6 voltsbias step.

In order to de-ionize the thyratrone tubes 85, 86 and 81, the outputfrom the plate of the limiter tube-fifi is applied `in parallel vthroughthe cfapacitors 120, 1)2! and `$22 to one side of the duo A P1590 voltspotential is applied as plate `potential through resistors H5, H6 andlli, respectively, in parallel to the plates of thethyratrons t5, 86 and8l. Variable adjustable potentiometer taps l2! and l22- introdueeadjustable resistances .to the plates of the duo diodes 9i and 93 on thevtube `side to which the +1500 volts is applied. `Step -voltageincrements, such as those shown in Fig. 3, are controlled by theposition on itsco'ntacts of the step voltage selector switch arm IDU,This application of the step voltage increments to the control .grids ofthe tubes Hand l2 allows an apparently simultaneous presentation on thescreen of the cathode ray tube .I5 of-two families of static curves thatare separately characteristic of and that permit the comparison-of thevacuum tubes il and I2. The described equipment 'provides control overthe magnitudes of bothg-rid and plate potentialsto be ,applied to theYtubes il and l2 to be compared.

In many instances it is desirable to be V,able to identify and to usevacuum vtubes of likelcharacteristics. 'Past methods used toidentify-'balanced tubes with certainty often consumed long periods oftime. With the c-omparator described herein, vacuum tubes may becompared Vand tested rapidly. The described comparator also `may be usedto provide a quick check on design current. The measure of thevariational or alternating current plate resistance is the ratio, for aconstant grid voltage, of a small plate voltage change to a small platecurrent change-it ell'ects. The measure of grid-plate transconductanceis the ratio, with a constant plate voltage,

vof a small change in plate current to a small change in grid voltagecausing the change in plate current.

Important characteristics of vacuum tubes having grid, plate and cathodeelectrodes are the relationships between plate current anad platevoltage with constant grid voltage; and plate current and grid voltagewith constant plate voltage. Curves showing plate current as a functionof grid voltage have shapes verysimilar to `those showing plate currentwith varying plate voltage, the difference being in the scales involvedand in the location of the curves with reference to the axes.Characteristic curves are commonly expressed as a relationship betweenplate current and plate voltage, with the grid voltage remainingconstant. The device d escribed herein provides visual presentations ofthese relationships.

In balanced detectors, balanced modulatorsdi rect current amplifiers andthe like, it is desirable to use vacuum Vtubes having likecharacteristics. The present invention serves to minimize the timerequired to identify vacuum tubes having like characteristics. Thepresent 4device makes theapplication of desired voltages-on the tubeelectrodes optional with a minimum time factor of that necessary tobring the tube up to its operating temperature. The device describedherein is adapted for use with all receiver type vacuum tubes, includingpower types. The present comparator graphically displays one or twofamilies of static characteristic curves simultaneously on the screen ofa cathode ray tube. With two vacuum tubes of like characteristics thecurves coincide.

In operating the equipment, it is rst brought up to its proper operatingtemperature, the arnplifier tubes 34 and 35 are balanced by the variableresistors 12 and 13, the electronic switch multivibrator is balanced bythe variable resistor B9, aand the tubes ll and I2 to be compared areplaced in their adapter sockets in the tube adapter 23. switch '21 inthe amplifier input selector 28 is caused to engage a contact connectedwith either the high or the low voltage generator or 26 and the platevoltage control in selector 28 is adjusted to provide the requiredamplitude. The amplifier gain variable resistors 12 and 13 are adjustedfor a proper amplifier gain control to give the required deflection. Thestep voltage selector switch arm |00 is then adjusted to the desiredvalue of grid voltage increments for application to the control grids ofthe tubes Il and l2 under test.

In making an amplifier balance adjustment, the tubes Il and l2 areremoved from their adapter sockets and the amplifier input selectorswitch 21 is placed in its balance position. The electronic switchbalance control is then adjusted until both amplier outputs from thetubes 34 and 35 start at the same level. The amplifier gain balancecontrol is then adjusted until the two amplifier outputs coincide. Inthe event an amplifier unbalance exists the two tube amplifiers arecaused to diverge from a common start.

It is to be understood that the circuitry and the associations shown anddescribed herein have been submitted for the purpose of presenting anoperable embodiment of the present invention and that similarlyfunctioning modifications may be made therein without departing from thescope of the present invention.

What we claim is:

l. A device for visually indicating for comparison the staticcharacteristic curves of plate voltage versus current, of a pair ofvacuum tubes comprising a cathode ray tube having a fluorescent Viewingscreen and two sets of beam deflecting means operative to deflect thebeam simultaneously in planes parallel to mutually perpendicularcoordinate axes respectively, a pair of plate circuits each having anidentical load resistance therein each circuit being operativelyassociated with one of said pair of vacuum tubes, a saw-tooth voltagewave generator connected in parallel with said plate circuits and one ofthe sets of cathode ray beam deflecting means, means for selectivelyvarying the peak Voltage of the saw-tooth output of said generator, apair of ampliers each operatively connected across the load resistanceof one of said plate circuits such that the amplifier output voltage isa measure of the current now in the associated plate circuit, anelectronic switching means connected to said ampliers and operative toalternately connect the amplier outputs to the other lSet of cathode raybeam deiiecting means and means for supplying a selected grid voltage tothe grids I of the vacuum tubes tobe compared.

The plate voltage 'selector' 2. A device for visually indicating forcomparison the static characteristic curves of plate voltage versuscurrent, of a pair of vacuum tubes comprising a cathode ray tube havinga fluorescent viewing Screen and two sets of beam deflecting meansoperative to deflect the beam simultaneously in planes parallel tomutually perpendicular coordinate axes respectively, a pair of platecircuits each having an identical load resistance therein each circuitbeing operatively associated with one of said pair of vacuum tubes, asaw-tooth voltage wave generator connected in parallel with said platecircuits and one of the sets of cathode ray beam deiiecting means, meansfor selectively varying the peak voltage of the saw-tooth output of saidgenerator, a pair of amplifiers each operatively connected across theload resistance of one of said plate circuits such that the amplifieroutput voltage is a measure of the current iow in the associated platecircuit, an electronic switching means connected to said amplifiers andoperative to alternately connect the amplifier outputs to other set ofcathode ray beam deflecting means, linear step voltage generator meanssupplying a voltage to the grids of said pair of vacuum tubes, andswitch means between said pair of vacuum tubes and said cathode ray tubefor timing the presentation on the screen thereof.

3. A device for visually indicating for comparison the staticcharacteristic curves of plate voltage Versus current, of a pair ofvacuum tubes comprising a cathode ray tube having a fluorescent viewingscreen and two sets of beam deflecting means operative to deflect thebeam simultaneously in planes parallel to mutually perpendicularcoordinate axes respectively, a pair of plate circuits each having anidentical load resistance therein each circuit being operativelyassociated with one of said pair of vacuum tubes, a saw-tooth voltagewave generator connected in parallel with said plate circuits and one ofthe sets of cathode ray beam defiecting means, means for separatelyvarying the peak voltage of the saw-tooth output of said generator, apair of amplifiers each operatively connected across the load resistanceof one of said plate circuits such that the amplifier output voltage isa measure of the current flow in the associated plate circuit, anelectronic switching means connected to said amplifiers and operative toalternately connect the amplifier outputs to the other set of cathoderay beam deecting means, linear step voltage generator means supplyingvoltage to the grids of said pair of vacuum tubes, a plurality ofthyratrons in said linear step voltage generator means that rireconsecutively during each sawtooth voltage from said saw-tooth voltagegenerator for applying a corresponding plurality of voltages in parallelto the grids of said pair of vacuum tubes.

4. The comparator dened in claim 3 wherein said saw-tooth voltage wavegenerator contains a frequency supplying oscillator comprising a pair ofthyratrons connected in series shunted by a capacitor to ground.

5. A comparator defined in claim 4 wherein an 1 adjustable grid biaspotential is supplied to one of the pair of oscillator thyratrons. f

WARREN P. MOLAND. HOWARD I-I. FERRIS.

No references cited.

